How to preallocate a datetime array in matlab – Matlab

The Problem:

In MATLAB, is it possible to pre-allocate a datetime array? The goal is to create a 10×100 datetime array to store dates read from multiple files. While it is straightforward to pre-allocate a numeric array using ‘zeros’, the same approach doesn’t seem to work with the datetime data type. Can you provide a solution or suggest an efficient method for this task?

The Solutions:

Solution 1: Use the `NaT()` Function

The `NaT()` function is a built-in function in MATLAB that stands for “not a time.” It can be used to create a `datetime` array with all values set to `NaT`. This is the most straightforward way to preallocate a `datetime` array, and it is also the most efficient.

To create a `datetime` array with all values set to `NaT`, you can use the following syntax:

DataTable = NaT(10, 100);

This will create a `datetime` array with 10 rows and 100 columns, with all values set to `NaT`.

Solution 2: Use the `repmat()` Function

The `repmat()` function can be used to create a new array by repeating an existing array a specified number of times. This can be used to preallocate a `datetime` array by repeating a single `datetime` value the desired number of times.

To create a `datetime` array with all values set to a specific value using the `repmat()` function, you can use the following syntax:

DataTable = repmat(datetime(0, 0, 0), 10, 100);

This will create a `datetime` array with 10 rows and 100 columns, with all values set to the `datetime` value `0000-01-01 00:00:00`.

Solution 2: Initialize an empty datetime array

Instead of preallocating a fixed-size datetime array, you can create an empty one and then populate it dynamically as needed. This provides more flexibility and allows your array to grow and shrink depending on the actual data you have.

To initialize an empty datetime array, use the following syntax:

DataTable = datetime([], [], []);

This will create an empty array of type datetime. You can then use various methods to add elements to the array, such as the `datenum` function to convert numeric dates to datetime values, or the `datetime` constructor to create datetime values from individual components.

For example, to add the current date and time to the array, you can use the following code:

DataTable(end+1) = datetime('now');

You can also use loops or indexing to populate the array with data from files or other sources.

This approach is more flexible than preallocating a fixed-size array, and it allows you to work with dynamic data sets without having to worry about resizing the array.

Solution 3: Using `NaT` for Default Value

Another approach to preallocating a datetime array in MATLAB is by utilizing the `NaT` ('not a time') value. `NaT` is a built-in MATLAB value that represents an invalid or missing datetime value. It has the advantage of being explicitly defined as a default value within the `datetime` datatype.

To preallocate a datetime array using `NaT`, you can directly initialize it with the `NaT` value. Here’s how you can do it:

DataTable = NaT(10, 100);

This will create a 10×100 datetime array with all elements initialized to `NaT`. You can then assign specific datetime values to the array elements as needed.

The benefit of using `NaT` is that it clearly indicates that the array elements initially contain invalid or missing datetime values. This can be useful for debugging and data validation purposes.

Solution 4: Preallocate a datetime array using cell array

To preallocate a `datetime` array in MATLAB, you can use a cell array. A cell array is a data structure that can store different types of data, including `datetime` objects. Here’s how you can do it:

1. Create a cell array using the `cell()` function:

“`
DataTable = cell(10, 100);
“`
This creates a cell array with 10 rows and 100 columns, where each element is initially empty.

2. Fill the cell array with `datetime` objects using curly braces ({}):

“`
DataTable{2, 3} = datetime(‘2015-12-02’);
“`
This assigns the datetime object representing December 2, 2015 to the cell in the second row and third column of the cell array.

You can continue to fill the cell array with `datetime` objects as needed. Once you have filled all the cells, you can access the `datetime` objects using the same curly brace notation:

date = DataTable{2, 3};

This will assign the `datetime` object representing December 2, 2015 to the variable `date`.

Using a cell array to preallocate a `datetime` array allows you to store a mix of `datetime` objects and other types of data in the same array.